Project Summary Down Syndrome (DS) is the most common genetic cause of intellectual disability (ID) in the United States. For a long time, researchers have considered spatial abilities as an area of strength in people with DS. As a core intelligence, spatial abilities are used in a wide variety of daily activities and serve as a foundation for other complex cognitive processes. Studying spatial abilities in people with DS has widespread implications, ranging from promoting their daily living skills, expanding their job opportunities, to providing key outcome measures for use in clinical trials. Despite the importance of the topic, our recent review suggests that it is too premature to conclude that spatial abilities, in general, are an area of strength for people with DS based on extant research. More specifically, previous research was seldom motivated by behavioral theories of spatial abilities, has yielded inconsistent results due to methodological variations, and has not assessed several key spatial abilities in people with DS. The current study proposes to systematically and comprehensively evaluate spatial abilities and behaviors in people with DS. Our rationale is that a systematic investigation of spatial abilities should be guided by a theoretical framework for categorizing spatial abilities and implemented through rigorous methodologies. Further, a complete spatial profile of people with DS requires both fundamental and applied approaches to research. We propose two specific aims: 1). To systematically evaluate the spatial abilities of individuals with DS relative to mental ability (MA)- matched typically developing (TD) children and MA- and chronological age (CA)- matched individuals with ID (not DS). 2). To determine the competencies in everyday spatial behaviors for people with DS and their relations with spatial abilities. To achieve these aims, 30 people with DS aged 16-25 years, 30 MA-matched TD children, and 30 MA- and CA-matched people with ID will participate in a series of spatial tasks representing four distinct spatial categories (i.e., intrinsic-static; intrinsic- dynamic; extrinsic-static; extrinsic-dynamic) based on a spatial theoretical framework. Their parents/caregivers will complete ratings of everyday spatial behaviors and daily living skills. Preliminary data support that people with DS may show more severe weaknesses in some, but not other, spatial areas and that there is an urgent need for research of spatial abilities voiced by the parents of children with DS. For the first time, this study will establish a comprehensive cognitive/behavioral profile of spatial abilities, thereby identifying the strengths and weaknesses within the spatial domain for individuals with DS. This study will also be the first to provide an ecological profile of spatial behaviors for individuals with DS. Practically, by identifying spatial weaknesses the proposed study will pave ways for clinical trials designed for improving the performance of persons with DS in spatial abilities and spatially oriented activities that are critical to everyday functioning and job prospects. Theoretically, the proposed study will help to identify the mechanisms of typical and atypical spatial development, and the roles of brain and neurological differences underlying DS.